The present invention relates generally to structural health monitoring methods and systems, and more particularly, to a structural health monitoring technique which actively interrogates the structure through broadband excitation of an array of piezoelectric transducers attached to or embedded within the structure for both actuation and sensing. Statistical analysis of the changes in transfer functions between actuator/sensor pairs is used to detect, localize, and assess the severity of damage in the structure.
A significant area of ongoing research and development efforts in the aerospace industry is the implementation of an automated structural health monitoring system (SHMS) using smart sensors and actuators integrated into the structure of an aerospace vehicle in order to provide a "built-in-test" (BIT) diagnostic capability for the structure. Such "smart structures" facilitate a reduction of acquisition and life cycle costs of aerospace vehicles which incorporate the same. A reliable SHMS will enable the practice of condition-based maintenance (CBM), which can significantly reduce life cycle costs by eliminating unnecessary inspections, minimizing inspection time and effort, and extending the useful life of new and aging aerospace structural components.
A principal requirement of an integrated SHMS is to provide a first level, qualitative damage detection, localization, and assessment capability which can signal the presence of structural damage and roughly localize the area where more precise quantitative non-destructive evaluation of the structure is needed.
Previous SHMS devices have primarily relied upon "passive" strain tracking or acoustic emission monitoring techniques, both of which have serious drawbacks and shortcomings. More particularly, both of these structural health monitoring techniques require continuous monitoring of the structure under evaluation in order to detect any damage to the structure. Thus, if a power failure (or power shut-down) occurs, the SHMS device is disabled. Further, the accuracy and reliability of the acoustic emission monitoring technique is compromised by the generally noisy environment of the aerospace vehicle. Another major disadvantage of acoustic emission (AE) monitoring is that a significant amount of data storage is required. The strain tracking technique requires the costly development of a finite element strain distribution model against which to compare the measured strain distribution across the structure in order to quantify and localize the damage. Moreover, both of these structural health monitoring techniques are severely limited with respect to their sensitivity, i.e., their ability to detect structural damage at the smallest possible level.
The most promising structural health monitoring techniques currently under development involve the use of piezoelectric transducers to actively excite and sense the vibration characteristics of the structure. This vibration signature is then compared with that of a normal undamaged structure and the difference is used to extract a metric related to the health of the structure. These structural health monitoring techniques can thus be regarded as "active" vibration-based structural health monitoring techniques.
Chaudhry et al. have demonstrated the use of piezoelectric transducers for local-area damage detection in metallic structure bond lines and joints, as well as in composite repair patches for metallic structures. See, Z. Chaudhry, T. Joseph, F. Sun, and C. Rogers, "Local-area health monitoring of aircraft via piezoelectric actuator/sensor patches,", Proceedings, SPIE Symposium on Smart Structures and Materials, Vol. 2443, pp. 38-49, 1995; and, Z. Chaudhry, F. Lalande, A. Ganino, and C. Rogers, "Monitoring the integrity of composite patch structural repair via piezoelectric actuators/sensors", Proceedings, 36th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, pp. 2243-2248, 1995, the disclosures of which are incorporated herein by reference. Their technique utilizes a single piezoelectric transducer made of Lead Zirconate Titanate (PZT) bonded to the structure to obtain electrical impedance measurements across a specific frequency range using simultaneous actuation and sensing. Since the PZT provides a coupling between mechanical impedance and electrical impedance, vital mechanical impedance information of the structure can be extracted from the electrical impedance measurements. Damage to the structure affects the vibration signature of the structure. Thus, damage can be detected by monitoring the appropriate frequency spectrum of the electrical impedance function. Chaudhry et al. have demonstrated that de-bonds as small as 1/4" in the composite patch produce a noticeable change in the impedance function at frequencies between 10 kHz and 20 kHz.
Castanien and Liang have extended this technique to include the cross-electromechanical impedance between pairs of PZT transducers to aid in monitoring a larger area of the structure while still providing some localization capability. See, K. E. Castanien and C. Liang, "Application of active structural health monitoring technique to aircraft fuselage structures", Proceedings, SPIE Symposium on Smart Structures and Materials, Vol. 2721, pp. 38-49, 1996, the disclosure of which is incorporated herein by reference. They have used this technique to demonstrate detection and localization of rivet line failures in a section of metallic aircraft fuselage using a frequency spectrum of 0 to 2 kHz.
Another "active" vibration-based structural health monitoring technique is disclosed in U.S. Pat. No. 5,327,358. However, the technique disclosed in this patent requires a model of the structure under evaluation, and thus, requires a priori information about the structure (including structural material and geometric properties) for deriving the model parameters.
Although the above-described vibration-based "active" structural health monitoring techniques are promising, they do not provide any mechanism for processing the information they obtain in order to facilitate accurate quantitative assessment and localization of any detected damage to the structure under evaluation. Further, their detection sensitivity and localization capabilities are limited, e.g., with respect to composite structure delamination damage.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a model-independent structural health monitoring system and method which overcomes the above-described drawbacks and shortcomings of the presently available technology. The present invention fulfills this need in the art.